This invention relates to the treatment of a body of manganese-aluminum alloy that is initially nonmagnetic wherein a selected portion is selectively treated to form regions of high intrinsic magnetic coercivity.
Alloys of aluminum and about 65-75 weight percent manganese that are rapidly cooled from a temperature above about 1100.degree. C. to about 400.degree. C. transform to a nonmagnetic phase. The microstructure of such nonmagnetic alloys of manganese-alluminum is substantially orthorhombic. Tempering at a suitable temperature above about 450.degree. C. substantially converts the orthorhombic alloys to a different crystal structure that is ferromagnetic, and has high coercivity. An alloy tempered in this manner has a relatively high magnetic saturation of about 7000 Gauss and uniaxial magnetic anisotropy. Magnets with energy products as high as 7.0 megaGauss-Oersteds have been prepared from compositions of about 70 weight percent manganese, 30 weight percent aluminum, and less than 1 weight percent carbon, the carbon acting to stabilize the magnetic manganese-aluminum phase. Such magnets are approximately 40 percent stronger per unit weight than typical ferrite magnets. The manganese-aluminum alloys also have high mechanical strength compared to magnetic aluminum-nickel-cobalt or ferrite alloys, and the raw materials are relatively inexpensive.
There are applications where it is desirable to magnetize only a selected region of a nonmagnetic manganese-aluminum alloy body. For example, an ordered array of dots or lines, readily readable by a conventional electronic pickup could be used to permanently, invisibly, and indelibly mark articles for identification purposes. Larger manganese-aluminum bodies, such as arcuate pole pieces for cylindrically-shaped DC motors, could be selectively magnetized to tailor magnetic flux fields for maximum device performance. Selected microscopic portions of, e.g., of thin layer of a nonmagnetic manganese-aluminum alloy, could be selectively magnetized to serve as permanent read-only-memory (ROM) for a microprocessor or other computer device.